Raw FIR: add work-around for deeply nested String interpolations

Controversial (but prevents total kotlin test failing)
Related to KT-29222

compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/ConversionUtils.kt

@@ -279,6 +279,7 @@ internal fun Array<KtStringTemplateEntry>.toInterpolatingCall(
     val sb = StringBuilder()
     var hasExpressions = false
     var result: FirExpression? = null
+    var callCreated = false
     for (entry in this) {
         val nextArgument = when (entry) {
             is KtLiteralStringTemplateEntry -> {
@@ -301,13 +302,19 @@ internal fun Array<KtStringTemplateEntry>.toInterpolatingCall(
                 )
             }
         }
-        result = when (result) {
-            null -> nextArgument
-            else -> FirFunctionCallImpl(session, base).apply {
-                calleeReference = FirSimpleNamedReference(session, base, OperatorNameConventions.PLUS)
-                explicitReceiver = result
+        result = when {
+            result == null -> nextArgument
+            callCreated && result is FirFunctionCallImpl -> result.apply {
                 arguments += nextArgument
             }
+            else -> {
+                callCreated = true
+                FirFunctionCallImpl(session, base).apply {
+                    calleeReference = FirSimpleNamedReference(session, base, OperatorNameConventions.PLUS)
+                    explicitReceiver = result
+                    arguments += nextArgument
+                }
+            }
         }
     }
     return if (hasExpressions) result!! else FirConstExpressionImpl(session, base, IrConstKind.String, sb.toString())

compiler/fir/psi2fir/testData/rawBuilder/expressions/for.txt

@@ -41,7 +41,7 @@ FILE: for.kt
             lval <destruct>: <implicit> = <iterator>#.next#()
             lval x: <implicit> = <destruct>#.component1()
             lval y: <implicit> = <destruct>#.component2()
-            println#(String(x = ).plus#(x#).plus#(String( y = )).plus#(y#))
+            println#(String(x = ).plus#(x#, String( y = ), y#))
         }
 
     }